1. Field of the Invention
The present invention relates to a method for transmitting a signal in a mobile communications system, and particularly, to a method for transmitting a signal in an MIMO system using a plurality of transmit/receive antennas.
2. Background of the Related Art
Recently, IMT-2000 system based on a third generation partnership project (3GPP) of W-CDMA wireless specification has adopted a space time transmit diversity (STTD) as an open loop type transmit diversity technique. The STTD refers to a technique for achieving a diversity advantage through a space-time coding extending a channel coding usually applied in a time base to a space based.
The STTD can be applied to every downlink physical channels except a synchronization channel (SCH) of WCDMA, in which there is no need for feedback information transmitted from a receiving end to a transmitting end, and accordingly it is advantageous to have diversity of system performances according to variation of speed and a radio channel.
The STTD technique uses two transmit antennas and a single receive antenna, by which diversity advantages of space and time can be achieved together by simply performing a space-time coding with respect to symbols transmitted through the two antennas. For this purpose, the conventional STTD system requires two transmit antennas and a space-time coding block (namely, an STTD encoder) in order to code symbols therebetween transmitted through the two transmit antennas. The STTD is operated as can be seen from [Table 1].
TABLE 1Time tTime t + TAntenna 1s1s2Antenna 2−s2 *s1 *Here, the mark ‘*’ denotes a conjugate.
Referring to [Table 1], symbols to be transmitted are encoded in the STTD encoder and transmitted to Antenna 1 and Antenna 2, respectively, according to time sequence. The signals transmitted to the respective antennas go through independent channels different from each other. Assuming that a channel in a time t is identical to a channel in a time t+T (T refers to a symbol period), received signals r1,r2 of the receiving end can be shown in Equation (1) herebelow.r1=r(t)=h1s1−h2s2*+n1 r2=r(t+T)=h1s2+h2s1*+n2  Equation (1)
Here, h1=a1ejθ1 and h2=a2ejθ2 refer to channels between each transmit antenna and the single receive antenna, and n1 and n2 refer to additive white Gaussian noise (AWGN) in the receiving end. In addition, each channel h1 and h2 may be estimated from a pattern of a pilot signal transmitted from the respective transmit antennas.
Therefore, once combining the two received signals r1 and r2 therewith in the receiving end as shown in Equation (2), it is possible to obtain the same value as an MRC (Maximum Ratio Combining) method of a receiving diversity. On the basis of this, the transmission symbol can be estimated.{tilde over (s)}1=h1*r1+h2r2*=(a12+a22)s1+h1*n1+h2n2*{tilde over (s)}1=h1*r2−h2r1*=(a12+a22)s2+h1n2*−h2*n1  Equation (2)
Double STTD
A double STTD (hereinafter, refer to D-STTD) is a method obtained by enlarging the STTD technique using only two transmit antennas to an MIMO (Multi Input Multi Output) system. Here, the D-STTD system using four transmit antennas can include two STTD pairs.
The D-STTD system has been first proposed in the 3GPP by Texas Instrument. After then, an antenna shuffling has been also proposed. In this antenna shuffling, different modulation methods (QPSK, QAM and the like) are applied to each STTD pair and the receiving end measures correlation between transmit antennas, thereby determining an STTD antenna pair. Furthermore, Mitsubishi Electric has been once proposed Sub group rate control D-STTD capable of applying an MCS (modulation and coding set) different from each other to the each STTD pair.
FIG. 1 is a schematic diagram showing a D-STTD system of the related art.
With reference to FIG. 1, data to be transmitted through the D-STTD system is separated in a demultiplexer 10 as a signal for two STTD pairs (each STTD pair includes a modulator, an STTD encoder and an antenna). The modulators 11 and 12 and the STTD encoders 13 and 14 of each STTD pair respectively modulate and code a signal by using a modulation and a coding rate selected depending on the MCS feedbacked from the receiving end. The transmitted signal passed through the respective modulators 11 and 12 and the respective STTD encoders 13 and 14 is transmitted through the transmit antenna pair. During this, the respective STTD encoders 13 and 14 receive two symbols at once from each modulator 11 and 12.
STTD decoders 15 and 16 and demodulators 17 and 18 of the receiving end decode and demodulate the signal received through the transmit antenna by using the MCS. The received signal demodulated by the respective demodulators 17 and 18 is then multiplexed at a multiplexer 19 and thereby recovered to the original data.
On the other hand, a channel predictor 20 of the receiving end receives the received signal and estimates a D-STTD channel matrix. An MCS selecting unit 21 calculates a signal to interference noise ratio (SINR) of each data stream on the basis of the estimated D-STTD channel matrix, and accordingly selects a proper MCS corresponding to the SINR of each data stream, thereafter feedbacking it to the transmitting end.
STTD Combined with Beamforming
FIG. 2 is a schematic diagram showing an STTD system combined with a beamforming of the related art. As can be seen from FIG. 2, the D-STTD system additionally includes beamforming units 22 and 23, which can be applied when using only two transmit antennas.
Referring to FIG. 2, first, data (a signal) to be transmitted is demultiplexed at the demultiplexer 30 and then inputted to each modulator 31 and 32. Each modulator 31 and 32 and each STTD encoder 33 and 34 respectively modulate and code the demultiplexed signal according to a modulation and a coding rate selected with reference to an MCS feedbacked from the receiving end. During this, the STTD encoders 33 and 34 receive two symbols at once from the respective modulators 31 and 32.
Beamforming units 35 and 36 perform a beamforming by multiplying a symbol (e.g., s1 and s2) which is space-time coded (STTD-performed) at each STTD encoder 33 and 34 by an eigenvector, and thereafter transmit it through each transmit antenna. In this case, the following Equation (3) will show a signal processing operation for performing the beamforming in addition to the STTD.
                                          [                                                                                s                    1                                                                                        s                    2                                                                                                                    -                                          s                      2                      *                                                                                                            s                    1                    *                                                                        ]                    ⁡                      [                                                                                w                    1                                                                                        w                    2                    *                                                                                                                    w                    2                                                                                        -                                          w                      1                      *                                                                                            ]                          ⁡                  [                                                                                          1                    +                    β                                                                              0                                                                    0                                                                                  1                    -                    β                                                                                ]                                    Equation        ⁢                                  ⁢                  (          3          )                    
Here, [w1 w2]T refers to an eigenvector corresponding to a maximum eigenvalue of a half-square matrix RT1/2 of a correlation matrix between transmit antennas. Additionally, β may be defined as
  β  =            (                        1                      λ            2            2                          -                  1                      λ            1            2                              )        /          (                        E          s                          σ          2                    )      in case of using two transmit antennas. At this time, the λ1 and λ2(λ2>λ1) refer to eigenvalues of the RT1/2. Also, Es and σ2 depict symbol energy and Gaussian noise, respectively.
Therefore, because the receiving end has already known of the MCS and eigenvector values w1 and w2, the respective STTD decoders 37 and 38 and the respective demodulators 39 and 40 decode and demodulate the received signal with reference to the MCS and the eigenvector. Then, the received signal demodulated at the respective demodulators 39 and 40 is converted into a serial data item at a multiplexer 41, and accordingly restored to an original data stream.
During this, the channel predictor 42 of the receiving end receives the received signal and estimates an STTD channel matrix. An MCS selecting unit 43 then calculates a signal to interference noise ratio (SINR) of each data stream depending on the estimated STTD channel matrix, and then selects an MCS corresponding to the SINR of each data stream, thereafter feedbacking it to the transmitting end.
As aforementioned, in the D-STTD system of the related art, in case that there are more than four transmit antennas therein, the transmit antennas are bound as two antenna pairs, and a plurality of data streams are transmitted through the corresponding antenna pair.
However, there exists a degree of correlation between the transmit antennas, it is not preferable to simply set the two adjacent antennas as a pair for an STTD encoding. Therefore, once obtaining the correlation between the transmit antennas directly in the receiving end, information to determine a transmit antenna pair can be feedbacked to the transmitting end. However, in case of using this method, it is very difficult for the receiving end to obtain the correlation between the transmit antennas, and accordingly the method may also be actually difficult to be applied.
In addition, as aforementioned, in the STTD system combined with the beamforming of the related art, in case that there are two transmit antennas, after performing the beamforming by multiplying the STTD-encoded data by the eigenvector, it is transmitted through each transmit antenna. However, this method for transmitting a signal is a signal processing method adopted only in case of using two transmit antennas, so that it is hard to be applied as it is when using more than two transmit antennas.
Furthermore, in the method for transmitting a signal combined with the beamforming, the eigenvector corresponding to the maximum eigenvalue of the half-square matrix of the correlation matrix (configuring the correlation value between the transmit antennas as a matrix) is used as a weight value for the beamforming. However, obtaining the value of the correlation matrix of the transmitting end is actually very difficult, so that the STTD combined with the beamforming of the related art can not be easily applied.
That is, in the related art, in case of using two transmit antennas in the STTD system, the eigenvector has been used as the weight value to be used for the beamforming in condition that it is possible to obtain the eigenvector corresponding to the maximum eigenvalue of the correlation matrix configured with the correlation value between transmit antennas.
By the way, because the channel matrix measured by the receiving end includes the correlation value between transmit antennas, the correlation value between receive antennas, and a channel value in a radio channel circumstance all together, it is actually very hard to separate only the correlation value between the transmit antennas from the channel matrix. Furthermore, the correlation value between the transmit antennas in an actual circumstance can be varied depending on time. The method for obtaining the weight value by using the correlation value between the transmit antennas of the related art may thus be hard to be actually applied.
Moreover, in case that there are one or more receiving end antennas, the correlation value between the receiving end antennas should be considered, but the method for transmitting a signal of the related art only considers the correlation between transmit antennas. As a result, it is not an actually proper method for application.